In recent years, for wireless communication systems such as cellular phone systems (cellular systems), next generation wireless communication techniques have been discussed in order to, for example, further increase the speed of wireless communication and enlarge the capacity of wireless communication. For example, 3rd Generation Partnership Project (3GPP), which is a standards organization, has suggested a communication standard, which is referred to as Long Term Evolution (LTE) and a communication standard, which is referred to as LTE-Advanced (LTE-A) that is based on wireless communication techniques of LTE. In the following, unless otherwise mentioned, “LTE” includes LTE and LTE-A and in addition other wireless communication systems in which those are expanded.
In the wireless communication system based on LTE, a transmission scheme which uses multiple antennas is specified. Such a transmission scheme is generally categorized into three of multiple input multiple output (MIMO), multiple input single output (MISO), and single input multiple output (SIMO). MIMO is correspondent to a case where each of a transmission device and a reception device uses plural antennas. As one example of MIMO, a case where the transmission device includes four transmit antennas and the reception device includes two receive antennas is generally expressed as “4×2 MIMO”. Meanwhile, MISO is correspondent to a case where only the transmission device uses plural antennas, and SIMO is correspondent to a case where only the reception device uses plural antennas.
Those multi-antenna techniques use properties in which the transmission-reception situation of wireless signals largely changes due to only a little difference in the positions of the antennas. Thus, in the multi-antenna techniques, plural communication paths (generally referred to as multipath) are formed between the transmission device and the reception device. In one example, in a case of 4×2 MIMO, 4×2=8 communication paths are formed, and multipath communication by those eight communication paths may be realized. In such multipath communication, plural communication paths are utilized, and wireless communication are thereby simultaneously (in parallel) executed by using the same frequency band. Consequently, by the multi-antenna technique, various effects may be obtained, which may not be obtained by wireless communication by a single transmit-receive antenna. For example, use of a limited wireless resource enables an improvement in the capacity (throughput) of wireless communication and expansion of the coverage.
Thus, also in 3GPP, researches and developments that are related to the multi-antenna techniques have been actively conducted. The newest specification of LTE supports a maximum of eight antennas for a wireless base station (evolved node B: eNB) and supports a maximum of four antennas for a wireless terminal (user equipment: UE). It may be expected that the number of usable antennas will further increase in the future in order to intend a further improvement in the throughput or the like.
Incidentally, in LTE, scheduling of wireless communication is in principle performed by the wireless base station. Here, the scheduling of wireless communication is allocation of wireless resources (frequencies and time) for performing wireless communication in a narrow sense but includes a variation of settings and designation of parameters for performing wireless communication in a broad sense. For example, selection of modulation and coding schemes that are used for wireless communication, selection of whether or not MIMO is performed, and so forth are portions of the scheduling, and those are performed by the wireless base station in LTE.
In order to schedule efficient wireless communication, the wireless base station has to recognize the state of a wireless communication path (channel) that is formed between the wireless base station itself and the wireless terminal. For example, in a case where the channel state is comparatively good, high-rate modulation and coding schemes are selected, and the throughput of wireless communication may thereby be enhanced. On the other hand, in a case where the channel state is comparatively bad, low-rate modulation and coding schemes are selected, and the accuracy of wireless communication may thereby be secured. In such a manner, in efficient wireless communication, it may be said that recognition of the channel state by the wireless base station is a premise.
By the way, wireless communication is generally performed bidirectionally between the wireless base station and the wireless terminal. The direction from the wireless terminal to the wireless base station is referred to as uplink, and the direction from the wireless base station to the wireless terminal is referred to as downlink.
Here, because the wireless base station serves as the reception device in uplink wireless communication, the wireless base station itself may measure the uplink channel state based on the reception signal. However, because the wireless base station serves as the transmission device in downlink wireless communication, the wireless base station itself may not measure the downlink channel state.
Accordingly, in LTE, uplink feedback information to the wireless base station based on the downlink channel state measured by the wireless terminal is defined. Those pieces of feedback information are referred to as channel state information (CSI). The CSI enables the wireless base station to recognize the downlink channel state to some extent, and the wireless base station is capable of performing adequate scheduling even in the downlink wireless communication.
Note that there are the CSI that notifies information which is decided based on the downlink channel quality and the CSI that notifies information which is decided based on the downlink channel characteristic. Here, although there are several indices that indicate the channel quality, representative indices are a signal interference noise ratio (SINR) and so forth. On the other hand, the channel characteristic is a characteristic itself of the channel and is specifically the change in the phase and amplitude of the wireless signal provided by the channel. In general, in a case of M×N MIMO, the channel characteristic is expressed by an M×N complex matrix (channel matrix).